Hello
I am trying to perform a fixed effect analysis on student achievement gain scores. My data is nested panel data that is two-level data as I have gain scores of students within classrooms. I have 1-2 gain scores for students, and there are 20-30 students per classroom. My data is such that:

class_room student_id . gain_score gain_year
1 1 2 1
1 2 3 1
1 3 2 1
2 1 2 2
2 4 1 1
2 5 2 1
2 2 2 2

My data is unbalanced as I only have records for the students in the classrooms and they may proceed in these schools or drop-out. Additionally because the students are nested within the classrooms they would naturally be repeated measures within the classrooms. I used the code: xtset class_room gain_year and received the error: repeated time values within panel. I am not sure how best to proceed.Any suggestions would be greatly appreciated.